But in the not-so-perfect real world, longer lasting ingredients and shorter monthly breaks between active pills could prove useful in bolstering this form of birth control, hints a new study from Germany.

A typical monthly regimen of oral contraceptives includes 21 days of active pills, usually containing a combination of the hormones estrogen and progesterone, and a 7-day break of inactive sugar pills. The intent is to prevent pregnancy by obstructing ovulation, or the release of an egg during a woman's monthly cycle.

"Forgetting the first active pills of a treatment cycle has the biggest impact on contraceptive failure as this time interval may allow the process that leads to ovulation to start," Dr. Jurgen Dinger of the Berlin Center for Epidemiology and Health Research told Reuters Health in an e-mail.

To test whether decreasing the pill-free interval and using hormones with a longer duration of action may help counteract this common problem, Dinger and his colleagues studied more than 52,000 U.S. women who took part in the "International Active Surveillance of Women Taking Oral Contraceptive" study.

The participants' average age was 26 years, and each had started on a new contraceptive. More than 80 percent of the women had previously taken another version of the Pill.

The researchers did not interfere with the choice of oral contraceptive, but rather categorized the women into three groups based on their new hormone regimens: 21 days of 3 milligrams of drospirenone (progesterone) plus 30 micrograms of ethinyl estradiol (estrogen); 24 days of 3 milligrams of drospirenone plus 20 micrograms of ethinyl estradiol, or another regimen.

The 21-day drospirenone regimen is currently sold under the brand name Yasmin and the 24-day regimen, with longer-lasting progesterone, is sold as Yaz. Generic versions are also available. Bayer Schering Pharma, maker of Yasmin, supported the study.

Over the course of 3 years following the women, more than 1,600 became pregnant unintentionally. About 1,400 of these contraceptive failures were due to an imperfect use of the Pill.

Unintended pregnancy rates for the 21-day and 24-day drospirenone regimens were 2.8 percent and 2.1 percent at the end of the first year, respectively, and 5.7 and 4.7 percent after the third year.

Women older than age 30 were significantly less likely than younger women to get pregnant unintentionally. Women with a college education and who had never carried a child were also less likely to become pregnant.

Obese women, who made up nearly a quarter of the U.S. women studied, had about a 50 percent greater risk of contraception failure after accounting for age, education level and whether or not they had prior children.

According to the researchers, under all the situations studied, the 24-day drospirenone regimen yielded more protection against pregnancy than the 21-day drospirenone regimen, as well as the other oral contraceptives.

However, compared to findings from the same study conducted in Europe, U.S. women showed an overall four-fold higher unintended pregnancy rate.

Dinger suggested that multiple factors could explain this gap in effectiveness, from higher rates of obesity in the U.S. to cultural, behavioral and genetic differences between the regions.

Dinger also noted that most programs that have tried to improve the proper use of the Pill have failed. "Our approach," he emphasized, "did not intend to investigate ways to improve compliance but to look at whether specific oral contraceptive regimens are more 'robust' against incompliance than others."

"This should not mitigate the need for good counseling of female adolescents and women intending to use the Pill," added Dinger. "A highly effective oral contraceptive regimen in conjunction with good counseling is the only way to minimize unintended pregnancies."